The present invention is directed to polyaxial bone screws for use in bone surgery, particularly spinal surgery. Such screws have a head that can swivel about a shank of the bone screw, allowing the head to be positioned in any of a number of angular configurations relative to the shank.
Many spinal surgery procedures require securing various implants to bone and especially to vertebrae along the spine. For example, elongate rods are often required that extend along the spine to provide support to vertebrae that have been damaged or weakened due to injury, disease or the like. Such rods are often supported by certain vertebrae and support other vertebrae. The most common mechanism for joining the rod to the spine is to implant bone screws into certain vertebrae which then in turn support the rod or are supported by the rod. Bone screws of this type may have a fixed head relative to a shank thereof. In the fixed bone screws, the head cannot be moved relative to the shank and the rod must be favorably positioned in order for it to be placed within the head. This is sometimes very difficult or impossible to do.
Polyaxial bone screws allow swivelling of the head relative to the shank to accommodate placement of the rod and are most commonly used. In particular, a polyaxial bone screw allows rotation of the head about the shank until a desired rotational position is achieved for the head relative to the shank, after which the rod can be laid or urged into a channel in the head. The position of the head is eventually locked with respect to movement relative to the shank by friction produced by combination of parts or other means before or after the rod is inserted in the head. Often, the rod is retained in the head and/or pressure is exerted against the rod by a closure top, plug or the like to capture the rod in the head of the screw.
Because such implants are for placement within the human body, it is desirable for the implant to have as little effect on the body as possible. Consequently, it is quite desirable for the implants to have a relatively small profile both in height and width. It is also desirable that the implants be lightweight.
One undesirable attribute of some of the swivel-head implants is the need for a multitude of components that may loosen or even disassemble within the body. It is most undesirable for components to be free to move around in the body after the completion of surgery. Loosening of components relative to each other may result in related undesirable movement of the bone or vertebra that the implant was intended to stabilize. Consequently, it is desirable for there to be a lightweight, low profile polyaxial bone screw which assembles in such a manner that subsequent pieces lock preceding pieces within the overall structure, so that there is less likelihood that the various pieces of the structure will undesirably disassemble.